CodeKitHubWhat Is This Tool?
Body Mass Index (BMI) is a simple screening measure calculated by dividing weight in kilograms by height in meters squared (BMI = kg / m²). Developed in the 19th century as a population-level statistic, it's now widely used as a quick, free way to flag whether someone's weight falls in a range associated with higher health risk.
BMI is a screening tool, not a diagnostic one. It doesn't distinguish muscle from fat, doesn't account for where fat is distributed, and the standard ranges were derived mostly from adult populations — which is why doctors pair it with other measurements (waist circumference, body composition, medical history) before drawing any conclusions.
Why Use It?
- Instant BMI calculation from height and weight, with support for both metric (cm, kg) and imperial (feet/inches, pounds) units.
- A visual scale shows exactly where your number falls across the underweight, normal, overweight and obese ranges — not just a raw figure.
- Uses the standard WHO adult BMI categories, the same thresholds used on most medical intake forms.
- Free, instant, and works on your phone — nothing is uploaded, the calculation runs entirely in your browser.
- A clear disclaimer keeps the result in context: BMI is a screening number, not a diagnosis.
How to Use
- Choose your preferred height unit (centimeters, or feet and inches) and enter your height.
- Choose your preferred weight unit (kilograms or pounds) and enter your weight.
- Press the calculate button.
- Read your BMI value, its category, and where it sits on the visual scale.
Example
Input
Height 175 cm, weight 70 kgOutput
BMI 22.9 — Normal weight70 ÷ (1.75 × 1.75) = 22.86, rounded to 22.9. This falls inside the WHO "normal weight" range of 18.5–24.9.
WHO BMI categories for adults
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
Who BMI doesn't work well for
- Pregnant women — weight gain during pregnancy is expected and healthy, so standard BMI ranges don't apply.
- Athletes and very muscular people — muscle is denser than fat, so a lean, muscular body can register as "overweight" on the BMI scale despite low body fat.
- Children and teenagers — growing bodies need age- and sex-specific growth charts, not the adult BMI scale.
- Older adults — muscle loss with age can mask a high body-fat percentage behind a "normal" BMI number.
- This calculator is a screening tool for general reference only. It is not a substitute for professional medical advice, diagnosis or treatment — talk to a doctor about what a healthy weight range looks like for you specifically.
Where this fits in real workflows
BMI is one input among several when tracking weight-related health goals. If you're monitoring change over time rather than a single reading, note the date alongside each measurement — the Date Calculator can help you track and compare intervals between check-ins.
Working out a daily calorie or macro target instead of a screening number? That's a different calculation entirely and isn't covered by BMI, which only compares height and weight.
Frequently Asked Questions
How is BMI calculated?
BMI = weight in kilograms ÷ (height in meters)². For imperial units, weight and height are first converted to kilograms and meters before applying the same formula, so the result is identical either way.
What are the BMI categories?
Using the WHO adult scale: below 18.5 is underweight, 18.5–24.9 is normal weight, 25–29.9 is overweight, and 30 or above is obese. These thresholds apply to adults and are not adjusted for age, sex or body frame.
Is BMI accurate for everyone?
No. BMI does not distinguish muscle from fat, so muscular athletes are often classified as overweight despite low body fat. It is also not designed for pregnant women, children, teenagers, or people over 65, whose healthy ranges differ from the standard adult scale. Always discuss your results with a doctor rather than relying on BMI alone.
Is this a medical diagnosis?
No. BMI is a population-level screening statistic, not a diagnostic tool. It cannot tell you about body fat percentage, fat distribution, or overall health. If you have concerns about your weight or health, consult a doctor or qualified healthcare provider — they can order tests and measurements that BMI alone cannot provide.
Is my data stored anywhere?
No. The calculation runs in your browser with JavaScript; your height and weight are never sent to a server, stored or logged.